Electric power transmission and distribution systems or networks comprise high-voltage tie lines for connecting geographically separated regions, and substations for transforming voltages and for switching connections between lines. Power generation and load flow in a network with several substations is controlled by a central energy management system. An important issue in the control of a power generation and load flow is to keep the network stable, i.e. to avoid voltage collapses and swings. A method for assessing network stability, based on voltage margins, is described in EP 1 217 709. This patent application and articles cited therein describe a “Voltage Instability Predictor” (VIP) that measures currents and voltages locally in order to infer a proximity to voltage collapse. The concept of the VIP is based on an equivalent network as shown in FIG. 3. One part of an electric power system is treated as a power source, another part as a load. The power source is represented by its Thévenin equivalent 1 with a source voltage Ēth and a Thévenin or source impedance Zth. An apparent load impedance Z1, represents the load. Both the Thévenin impedance Zth and the apparent load impedance Z1 are estimated from the current and voltage measurements ī2, v2 by a VIP device. The relation of these impedances, expressed by a stability margin or power margin, indicates how close the power system or network is to collapsing.
This approach uses measurements taken at a single load bus. A single set of measurements does not contain enough information to directly compute the parameters of the Thévenin equivalent Ēth, Zth, however these can be estimated using for example using the least-squares method once at least two sets of measurements made at different times are available. For accurate estimation of these parameters, a sufficient change in the measurements must have occurred, e.g. due to load change. During this time the generation and transmission network is assumed to remain constant, which introduces an estimation error. Furthermore, the estimation is noise-sensitive and introduces a time-delay. The practical applicability of the method is therefore limited.
Other approaches are based on a complete set of system information, that is, all voltages and currents are measured, and the impedances of all lines and loads and generator capability diagrams are measured or estimated. Although these techniques are powerful tools capable of very accurately assessing the voltage stability of the network, the large number of measurements required makes them expensive and impractical to implement in practice.
It is therefore desirable to provide improved and efficient means to determine a model of a section of an electrical network, in particular for a section that essentially serves for transmitting power from a generating section to a load section.